Field of the Invention
The invention relates to a method for controlling a VAr compensator which is connected to an AC voltage which has a plurality of phases, in which an actual voltage, which is dropped across each phase, is determined for each phase, and a negative phase-sequence system actual component is calculated from the actual voltages, and in which a control loop suppresses the negative phase-sequence system actual component, wherein a feedback loop determines the degree of suppression of the negative phase-sequence system actual component with the aid of control parameters.
A method such as this is already known from routine practice in the field of power transmission and distribution and is used, for example, in conjunction with the control of a so-called static VAr compensator (SVC). An SVC is generally used to stabilize and balance the voltages in a power distributor system. For this purpose, inductances or capacitances are connected in parallel with the power supply system, by means of expedient switching units. Semiconductor switches, in particular, are provided as switching units, which prevent or allow current flow via said inductance or capacitance. An SVC can therefore be used to suppress a so-called negative phase-sequence system, by balancing phase voltages.
FIG. 1 schematically illustrates a method for controlling an SVC, by means of which a negative phase-sequence system is suppressed or partially allowed. For this purpose, voltage sensors detect the voltage which is dropped across the phases and provide phase voltage measured values v1,2, v2,3 and v3,1. The phase voltage measured values v1,2, v2,3 and v3,1 obtained in this way are supplied to a difference voltage production unit 1, which produces difference voltage ΔvD12, ΔvD23 and ΔvD31 at its output. The difference voltages can be calculated in various ways. Merely by way of example, reference is made to the discrepancy between the phase voltage measured value and a mean value of the phase voltage averaged over all three phases. Furthermore, a reference voltage vref is supplied to the difference voltage production unit 1, and likewise plays a role in determining the difference voltages. For example, the mean value of the phase voltage measured values is compared with the reference voltage, with the difference calculated in this way being applied to all phases. The difference voltages obtained in this way are each supplied to subtractors 2 which, at their output, produce the difference between the difference voltages AvD and the output of a feedback loop 3, resulting in corrected difference voltages Δv12, Δv23 and Δv31. The feedback loop 3 will be described in more detail later.
The corrected difference voltages Δv are each supplied to a voltage regulator 4 which, at its output, produces reactive-power control variables Qreg12, Qreg23 and Qreg31, which are respectively converted by a conversion unit 5 to susceptance values B12, B23 and B31. The susceptance values B12, B23 and B31 or the reactive-power control variable Qreg are/is used as a control variable of the SVC. If the output of the feedback loop 3 is equal to zero for all phases, the control system ensures that a voltage drop in the phase 1 and constant voltages in phases 2 and 3 result in a negative difference voltage value ΔDv12. The negative difference voltage value ΔDv12 produces a greater reactive-power control variable Qreg1,2 on the output side of the voltage regulator 4 than for the remaining phases 2 and 3. In comparison to the remaining phases, the SVC results in an increased feed-depending on the operating point of the SVC—of capacitive reactive power, or reduced feed of inductive reactive power, thus compensating for the voltage drop in the phase 1. The negative phase-sequence system is suppressed in this way. The negative phase-sequence system is calculated by a negative phase-sequence system calculation unit 6, which is likewise supplied with the phase voltage measured values v12, v23 and v31, and is displayed to a user.
However, without the influence of the feedback loop 3, a control method such as this is susceptible to errors. For example, if a single phase of the connected power distribution line fails totally, it is no longer possible to suppress the negative phase-sequence system without completely interrupting the power transmission. The feedback loop 3 is therefore used to limit the suppression of the negative phase-sequence system. For this purpose, the mean value of the susceptance values is first of all formed with the aid of an adder 7 and a divider 8, with difference susceptance values ΔB12, ΔB23 and ΔB31 being determined with the aid of the subtractor 9. These difference susceptance values are then each supplied to a feedback regulator 10 which, at its output, produces feedback difference voltages ΔvB12, ΔvB23 and ΔvB31 which are each supplied to the subtractor 2 as a second input variable. In this case, each feedback regulator 10 is limited at the top and bottom by an upper and a lower limit, which are defined by means of a limit preset unit 11. The degree of suppression of the negative phase-sequence system can be adjusted by adjusting the upper and lower limits, which are otherwise the same for all phases.
If, for example, the limits of each feedback regulator 10 are set to zero, the effect of the feedback loop 3 is increased. The suppression of the negative phase-sequence system is allowed without any impediment or restriction. In contrast, if the limits of each regulator are set to infinity, the negative phase-sequence system is not suppressed at all. The negative phase-sequence system is therefore allowed as it is. There is no balancing of the phase voltages.
The abovementioned control method has the disadvantage that the upper and lower limits of the feedback regulator 10 must be entered manually, that is to say for example by the personnel at a control center. However, this is complex and can lead to damage, particularly if the operator is careless.